Steel pipe having high formability and method for production thereof

ABSTRACT

The invention provides a steel pipe being superior in workability, particularly in bending workability, in which an r-value in the axial direction of the pipe in a portion where melting or transformation of a steel material has occurred during seam welding is as high as comparable to that in a portion where melting or transformation of the steel material has not occurred, and a method of producing the steel pipe. In the high-workability steel pipe, an r-value in the longitudinal direction is not less than 1.2, more preferably not less than 1.6, over an entire area in the circumferential direction, including a seamed portion. The steel pipe is produced by a method comprising the step of performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C. to Ac 3  with a reduction in diameter of not less than 30%, preferably after heating the steel pipe to temperatures of not lower than Ac 1 , the steel pipe being produced by seam-welding strip steel, or a method further comprising the step of performing heat treatment of holding the rolled steel pipe in a temperature range of from 600° C. to 900° C. for a time of 1 second or longer during cooling subsequent to the diameter-reducing rolling or by reheating the rolled steel pipe after the cooling.

TECHNICAL FIELD

[0001] The present invention relates to a steel pipe having superior workability and a method of producing the steel pipe.

BACKGROUND ART

[0002] For the purpose of reducing the weight and cost, the application of seam (electric resistance) welded steel pipes to automobile parts has been considered. Conventional seam welded steel pipes, however, have not been sufficient in workability. Bending is employed to manufacture, e.g., undercarriage or suspension parts of automobiles. When the conventional seam welded steel pipes are subjected to the bending, a problem has been experienced in that a pipe wall is greatly thinned on the outer side of a bent portion, and in the worst case a pipe is ruptured. Even in the case of not causing a rupture, a large rate of thinning of the pipe wall requires the use of a material having a greater thickness to satisfy the design stress, and therefore a sufficient reduction in weight cannot be achieved.

[0003] As disclosed in Japanese Unexamined Patent Application Publication No. 55-56624, for example, it is known that improving an r-value (Lankford value) of a pipe in the axial direction is effective to overcome the problems described above. As a method for increasing the r-value of a steel pipe, however, it is only known to increase the r-value of strip steel as a base material of a steel pipe as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 6-41689. When producing seam welded steel pipes, there has been a problem that the r-value is reduced in a portion where melting or transformation of a steel material has occurred during seam welding. Another problem has arisen in that the seam welding cannot be applied to steel plates not having a high r-value, such as hot-rolled steel plates, high tensile strength steel plates, and low, medium and high carbon steel plates.

[0004] Accordingly, it is an object of the present invention to provide a steel pipe being superior in workability, particularly in bending workability, in which an r-value of the pipe in the axial direction in a portion where melting or transformation of a steel material has occurred during seam welding is as high as comparable to that in a portion where melting or transformation of the steel material has not occurred, and a method of producing the steel pipe.

DISCLOSURE OF INVENTION

[0005] With the view of overcoming the problems mentioned above, the inventors have conducted studies based on a consideration that working and heat treatment of seam welded steel pipes are required to improve the r-value in a welded portion near the seam. Then, the inventors have studied a method of performing working and heat treatment of a steel pipe evenly at any positions in the circumferential direction, the steel pipe being produced by seam-welding cold-rolled steel having a high r-value. In the process of the studies, the inventors have found that the r-value of the seam welded steel pipe in the longitudinal direction (in the axial direction of the pipe) is noticeably improved to 1.2 or above, in particular to 1.6 or above, at any positions in the circumferential direction, including a seamed portion, by a method of performing diameter-reducing rolling on the seam welded steel pipe in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30% (referred to as a “method according to the present invention” hereinafter).

[0006] As a result of applying the method according to the present invention to seam welded steel pipes produced using various kinds of steel plates as base-material strip steel, the inventors have also found that a high r-value can be obtained regardless of the r-value of the original strip steel. Further, it has been found that with the method according to the present invention, the restriction of ingredients which has hitherto been employed to obtain a high r-value in steel sheets, i.e., a reduction of the C and N contents and addition of stabilizing elements such as Ti and Nb, are not required. As a result, seam welded steel pipes having a high r-value can also be produced using, as base-material strip steel, hot-rolled steel, high tensile strength steel such as dual phase steel, and low, medium and high carbon steel, which have a difficulty in achieving a high r-value in the stage of strip steel.

[0007] The views of the inventors regarding the reason why a steel pipe having a high r-value can be obtained from even a steel plate not having a high r-value are as follows.

[0008] By performing the diameter-reducing rolling on a seam welded steel pipe in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30%, an ideal aggregation structure due to the rolling, in which the <110> axis is parallel to the longitudinal direction and the <111 > to <110> axes are parallel to the radial direction, is formed and then further developed through restoration and recrystallization. That aggregation structure provides a high r-value. The aggregation structure due to the rolling produces very great driving forces because crystals are rotated by working strains. Unlike an aggregation structure that is created through recrystallization in the case of obtaining a high r-value in steel sheets, the aggregation structure due to the rolling is less affected by the second phase and solid solution C. Consequently, even for the type of strip steel which has a difficulty in obtaining a high r-value in the stage of producing steel plates, a high r-value can be obtained in the stage of producing steel pipes.

[0009] Also, the reason why a high r-value is not obtained by performing the diameter-reducing rolling at low temperatures is that ideal crystal rotation is not caused because of high work hardness, or that restoration and recrystallization are not developed at a sufficient level because of low temperatures. Furthermore, the reason why a high r-value is not obtained by a method of performing the diameter-reducing rolling on a steel pipe at low temperatures and then annealing the rolled steel pipe for recrystallization is that the desired aggregation structure is not developed through the cold rolling and the recrystallization because of the effect of the second phase and solid solution C.

[0010] In the field of producing steel sheets, there is known a method of producing a steel sheet having a high r-value by rolling steel into a sheet in the hot ferrite range. This method of producing a steel sheet having a high r-value is featured in that steel containing C and N in reduced amounts and added with stabilizing elements such as Ti and Nb is rolled at low temperatures and then recrystallized. That sheet rolling at low temperatures differs from the diameter-reducing rolling at high temperatures intended by the method according to the present invention. In fact, if the known sheet rolling in the hot ferrite range is carried out at 600° C. or above, the r-value is not improved, but rather noticeably lowered on the contrary. This is because, in the sheet rolling in which draft is applied in the thickness direction of a sheet, strain occurs in a direction different from that in the diameter-reducing rolling of a steel pipe in which draft is applied in the circumferential direction, and hence the aggregation structure effective in increasing the r-value is not developed.

[0011] As a result of further continuing the studies, the inventors have found that, in the method according to the present invention, the thickness deviation can be noticeably reduced and the occurrence of wrinkles near the seam can be suppressed by heating a seam welded steel pipe to temperatures of not lower than Ac₁ before the diameter-reducing rolling for austenitic transformation of a part or the whole of a steel structure, because the difference in mechanical properties between the hardened structure of the seam and the remaining portion is reduced. The present invention has been accomplished based on the findings set forth above. The features of the present invention are as follows.

[0012] (1) A high-workability steel pipe wherein an r-value in the longitudinal direction is not less than 1.2, more preferably not less than 1.6, over an entire area in the circumferential direction, including a seamed portion.

[0013] (2) A method of producing a high-workability steel pipe, the method comprising the step of performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30%, the steel pipe being produced by seam-welding strip steel.

[0014] (3) A method of producing a high-workability steel pipe, the method comprising the steps of heating a steel pipe to temperatures of not lower than Ac₁, the steel pipe being produced by seam-welding strip steel, and then immediately or after cooling and reheating the steel pipe, performing diameter-reducing rolling in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30%.

[0015] (4) In the method of producing a high-workability steel pipe defined in the above (2) or (3), after the diameter-reducing rolling of the steel pipe, heat treatment of holding the rolled steel pipe in a temperature range of from 600° C. to 900° C. for a time of 1 second or longer is performed during cooling subsequent to the diameter-reducing rolling or by reheating the rolled steel pipe after the cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a graph showing the relationship between an r-value in the longitudinal direction of a steel pipe having been subjected to diameter-reducing rolling and a reduction in diameter.

[0017]FIG. 2 is a graph showing the relationship between an r-value in the longitudinal direction of a steel pipe having been subjected to diameter-reducing rolling and an outgoing-side temperature in the rolling process.

[0018]FIG. 3 is a graph showing the relationship between a seam thickness deviation in a steel pipe having been subjected to diameter-reducing rolling and a heating temperature before the diameter-reducing rolling.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] In a high-workability steel pipe according to the present invention, an r-value in the longitudinal direction is not less than 1.2. The reason is that the bending workability of the steel pipe is noticeably improved when the r-value is not less than 1.2. More preferably, the high-workability steel pipe has an r-value of not less than 1.6 because the bending workability is further improved when the r-value is not less than 1.6.

[0020] The high-workability steel pipe according to the present invention can be produced by performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30%, the steel pipe being produced by seam-welding strip steel and having a seam. The r-value is affected by the reduction in diameter and the temperature during the diameter-reducing rolling.

[0021]FIG. 1 is a graph showing the relationship between the r-value in the longitudinal direction and the reduction in diameter resulted at circumferential positions 0°, 90°, 180° and 270° of each steel pipe which was produced by performing the diameter-reducing rolling on a seam welded steel pipe under a condition of the outgoing-side temperature being set to 730° C. while changing the reduction in diameter the seam welded steel pipe being produced by an ordinary method from strip steel having the same composition as steel A in Table 1 given below. The seam position is assumed to be at 0° (this is similarly applied to the following description). From FIG. 1, it is understood that, regardless of the circumferential positions, the r-value of not less than 1.3 is obtained at the reduction in diameter of not less than 30%, and the r-value of not less than 1.6 is obtained at the reduction in diameter of not less than 50%.

[0022]FIG. 2 is a graph showing the relationship between the r-value in the longitudinal direction and the outgoing-side temperature resulted at circumferential positions 0°, 90°, 180° and 270° of each steel pipe which was produced by performing the diameter-reducing rolling on a seam welded steel pipe under a condition of the reduction in diameter set to 30% while changing the outgoing-side temperature, the seam welded steel pipe being produced by an ordinary method from strip steel having the same composition as steel A in Table 1 given below. From FIG. 2, it is understood that the r-value of not less than 1.2 is obtained at the outgoing-side temperature of not lower than 600° C.

[0023] Based on the experiment results mentioned above, a lower limit of the temperature for the diameter-reducing rolling was set to 600° C. and a lower limit of the reduction in diameter was set to 30%. Also, an upper limit of the temperature for the diameter-reducing rolling was set to the same as an upper limit of the temperature range in which the steel structure contains ferrite, i.e., the temperature Ac₃. The r-value is not improved even by the diameter-reducing rolling if it is performed on steel whose structure contains no ferrite. The temperature Ac₃ depends on the chemical composition of steel, and can be determined based on experiments. A range of temperature Ac₃ is approximately not higher than 900° C. In the present invention, so long as the steel structure contains ferrite, the second phase (phase other than ferrite) is not limited to particular one. For example, austenite may be the second phase. More preferably, the diameter-reducing rolling is performed at temperatures where ferrite forms the main phase (phase having a volume ratio of 50% or more).

[0024] The gist of the present invention resides in that a steel pipe is subjected to the diameter-reducing rolling in a temperature range where the steel structure has the ferrite phase. From the standpoint of improving the r-value, there is no particular restriction upon the history prior to the diameter-reducing rolling. For example, the heating temperature prior to the diameter-reducing rolling may be any of the temperature at which the steel structure has the single austenitic phase, the temperature at which the steel structure has the two austenitic and ferrite phases, and the temperature at which the steel structure has the single ferrite phase. Further, prior to the diameter-reducing rolling, the steel pipe may be rolled at such temperatures as forming austenite as the single phase or the main phase.

[0025]FIG. 3 is a graph showing the relationship between a heating temperature and a thickness deviation resulted for each steel pipe which was produced by performing the diameter-reducing rolling on a seam welded steel pipe under conditions of the reduction in diameter set to 30% and the rolling temperature set to 700° C. while changing the heating temperature, the seam welded steel pipe being produced by an ordinary method from strip steel having the same composition as steel A in Table 1 given below. From FIG. 3, it is understood that the heating prior to the diameter-reducing rolling is preferably set to be not lower than the temperature Ac₁ from the standpoint of suppressing the thickness deviation and wrinkles occurred near the seam. The temperature Ac₁ depends on the chemical composition of the steel pipe, etc., and can be determined based on experiments. A range of temperature Ac₁ is approximately not lower than 800° C. However, if the heating temperature is too high, the crystal grain size would be excessively increased, thus resulting in a problem of, for example, increasing surface roughness during the working. For that reason, the heating temperature is preferably set to be not higher than 900° C.

[0026] There is no particular restriction upon the cooling after the heating of the steel pipe. Subsequent to the heating, the diameter-reducing rolling may be performed, for example, after cooling the steel pipe down to temperatures at which ferrite forms the main phase, or by reheating the steel pipe after cooling it down to the room temperature.

[0027] Further, preferably, after the diameter-reducing rolling of the steel pipe, heat treatment of holding the rolled steel pipe in a temperature range of from 600° C. to 900° C. for a time of 1 second or longer is performed in the present invention.

[0028] In the present invention, since the diameter-reducing rolling is performed at temperatures of not lower than 600° C., the work hardness is low and a sufficient level of workability is obtained with additional treatment. Even so, by performing heat treatment for holding the rolled steel pipe at a certain temperature for a certain time in succession to the diameter-reducing rolling, the elongation and the r-value are further improved. This effect is developed by holding the rolled steel pipe at temperatures of not lower than 600° C. for a time of 1 second or longer. However, if the holding temperature exceeds 900° C., the steel structure would be transformed into the single austenitic phase and the r-value would be reduced because of the randomized aggregation structure. For that reason, the heat treatment is preferably performed on conditions of the holding temperature in the range of from 600° C. to 900° C. and the holding time of 1 second or longer. Additionally, the heat treatment may be performed during cooling subsequent to the diameter-reducing rolling or by reheating the rolled steel pipe after the cooling.

EXAMPLE

[0029] Seam welded steel pipes were produced by an ordinary method from various kinds of hot-rolled steel plates having chemical compositions shown in Table 1, and the diameter-reducing rolling was performed on each steel pipe under conditions shown in Table 2. Heating of the steel pipe prior to the diameter-reducing rolling was not held at all or held for a time of 1 to 600 seconds after reaching the temperature shown in Table 2. Tensile specimens of JIS No. 12-A were sampled from circumferential positions 0°, 90°, 180° and 270° of each steel pipe obtained. After bonding a strain gauge with a gauge length of 2 mm to each specimen, a tensile test was carried out on the specimen by applying a nominal strain of 6 to 7%. Then, a ratio of a true strain ε_(w) in the width direction to a true strain ε_(L) in the longitudinal direction was measured. From a gradient ρ of that ratio, the r-value was calculated based on the following formulae:

ρ=ε_(L)/ε_(w)

r-value=ρ/(−1−ρ)

[0030] Further, a thickness deviation η was calculated by measuring a pipe wall thickness ts of a seamed portion and an average pipe wall thickness tb of the remaining portion. That is:

thickness deviation η%=(ts−tb)/tb×100%

[0031] Moreover, the presence or absence of wrinkles was determined by observing an image of an area near the seam in a cross-section perpendicular to the axis of the steel pipe, the image being enlarged at a magnification of 50 times.

[0032] Those results are listed in Table 3 along with the tensile strength (TS) and the elongation (E1).

[0033] The r-value is 1.2 or above at any positions in the circumferential direction in Examples of the present invention, whereas the r-value is below 1.2 in Comparative Examples. Also, in the specimens heated to temperatures of not lower than Ac₁, the thickness deviation is smaller and wrinkles are not caused.

Industrial Applicability

[0034] According to the present invention, a high-workability steel pipe can be provided which has a high r-value over an entire area in the circumferential direction, including a seamed portion, and also has a good shape. Limits in bending and expanding work of the steel pipe are noticeably improved, whereby omission of steps due to the integral forming and a reduction in weight can be achieved. Further, seam welded steel pipes having a high r-value can also be produced using, as base materials, hot-rolled steel, high tensile strength steel such as dual phase steel, and low, medium and high carbon steel, which have a difficulty in achieving a high r-value with a conventional method of producing a steel pipe by simply seam-welding a steel plate. As a result, the present invention is able to remarkably enlarge the applicable range of bending of steel pipes and hence greatly contributes to development of the industry. TABLE 1 Chemical Composition (&) Ac₁ Ac₃ Steel C Si Mn P S Al N Cr Ti Nb B Ni Cu (° C.) (° C.) A 0.06 0.1 0.3 0.01 0.005 0.02 0.003 — — — — — — 730 840 B 0.1 0.2 0.8 0.01 0.005 0.02 0.003 — — — — — — 730 820 C 0.25 0.3 0.8 0.01 0.005 0.02 0.003 — — — — — — 750 800 D 0.25 0.3 0.5 0.01 0.005 0.02 0.003 — — — 0.002 — — 750 800 E 0.4 0.3 1.6 0.01 0.005 0.02 0.003  0.03 — — — — — 730 780 F 0.08 1.0 1.4 0.01 0.005 0.02 0.003  0.9 0.01 — — — — 750 840 G 0.15 1.4 1.5 0.01 0.005 0.02 0.003  0.3 — — — — — 770 820 H 0.08 0.5 1.2 0.01 0.005 0.02 0.003 — 0.04 — — — — 770 820 I 0.08 0.04 1.5 0.01 0.005 0.02 0.003 — 0.04 — — — — 750 800 J 0.08 1.5 1.8 0.01 0.005 0.02 0.003 — 0.1  — — — — 780 830 K 0.09 0.05 1.8 0.01 0.005 0.02 0.003 — 0.15 0.05 — — — 750 800 L 0.01 0.2 1.5 0.01 0.005 0.02 0.003 11.0 — — — 0.25 0.4 730 800

[0035] TABLE 2 Incoming-side Outgoing-side Total Effective Temperature Temperature Reduction Reduction Heating in Diameter- in Diameter- in in Temperature Reducing Reducing Diameter Diameter* Heat No. Steel (° C.) Rolling (° C.) Rolling (° C.) (%) (%) Treatment Remarks 1 A 800 780 730 50 50 — Example 2 A 900 880 830 50 5 — Comparative Example 3 A 630 610 560 50 10 — Comparative Example 4 B 800 780 730 50 50 — Example S B 800 780 730 50 50 — Example 6 C 800 780 730 50 50 730° C. × Example 5 min. 7 D 900** 720 680 50 50 — Example 8 D 850 720 680 50 50 — Example 9 D 800 780 730 50 50 — Example 10 D 800 720 680 50 50 — Example 11 D 750 720 680 50 50 — Example 12 D 735 720 680 50 50 — Example 13 D 720 720 680 50 50 — Example 14 E 800 780 730 50 50 — Example 15 F 800 780 730 0 0 — Comparative Example 16 F 800 780 730 15 15 — Comparative Example 17 F 800 780 730 30 30 — Example 18 F 800 780 730 40 40 — Example 19 F 800 780 730 50 50 — Example 20 F 800 780 730 60 60 — Example 21 F 800 780 730 70 70 — Example 22 F 900 890 850 30 2 — Comparative Example 23 F 850 840 780 30 30 — Example 24 F 750 730 680 30 30 — Example 25 F 700 680 600 30 30 — Example 26 F 630 610 560 50 10 — Comparative Example 27 G 900 780 730 50 50 — Example 28 G 850 780 730 50 50 — Example 29 G 800 780 730 30 30 — Example 30 G 800 780 730 40 40 — Example 31 G 800 780 730 50 50 — Example 32 H 800 780 730 50 50 — Example 33 I 800 780 730 50 50 — Example 34 J 800 780 730 50 50 — Example 35 K 800 780 730 50 50 — Example 36 L 760 740 700 60 60 — Example

[0036] TABLE 3 Wrinkles Seam ∘ not 0° (Seam) 90° 180° 270° Thickness occurred TS/ EI* r- TS/ EI* r- TS/ EI* r- TS/ EI* r- Deviation x No Mpa /% value Mpa /% value Mpa /% value Mpa /% value /% occurred Remarks 1 300 55 2.0 303 54 2.0 307 54 2.1 301 55 2.1 0.3 ∘ Example 2 300 45 0.8 309 45 0.9 307 45 0.8 308 45 0.8 0.3 ∘ Comparative Example 3 450 35 1.0 450 35 1.1 459 36 1.0 451 34 1.1 10.0 X Comparative Example 4 350 50 2.0 356 51 2.0 356 50 2.0 350 51 2.0 0.5 ∘ Example 5 350 50 2.4 358 51 2.4 351 49 2.5 356 49 2.4 0.5 ∘ Example 6 620 25 1.8 624 24 1.8 625 25 1.8 629 25 1.9 0.3 ∘ Example 7 640 27 1.7 646 27 1.7 641 27 1.7 647 26 1.7 0.5 ∘ Example 8 631 25 1.7 651 26 1.6 641 25 1.8 641 25 1.8 1.0 ∘ Example 9 620 28 1.8 626 29 1.8 621 29 1.9 627 28 1.9 0.5 ∘ Example 10 640 24 1.6 659 24 1.7 632 24 1.7 636 24 1.7 2.0 ∘ Example 11 644 22 1.6 650 22 1.7 635 22 1.7 632 22 1.8 30 ∘ Example 12 653 20 1.6 657 21 1.6 640 21 1.8 623 21 1.8 8.0 x Example 13 644 19 1.7 650 19 1.7 637 19 1.9 614 19 1.8 15.0 x Example 14 650 25 1.8 652 25 1.9 651 25 1.8 651 26 1.9 0.5 ∘ Example 15 500 25 0.7 508 26 0.8 503 24 0.8 SOl 25 0.8 0.3 ∘ Comparative Example 16 590 28 1.0 593 28 1.1 599 29 1.1 595 28 1.0 0.3 ∘ Comparative Example 17 610 28 1.3 610 28 1.3 618 28 1.3 614 29 1.3 0.9 ∘ Example 18 610 29 1.4 619 29 1.4 611 30 1.4 611 28 1.4 0.9 ∘ Example 19 610 30 1.6 6l7 31 1.7 611 30 1.6 61S 31 1.6 0.9 ∘ Example 20 610 32 2.0 616 31 2.0 612 33 2.1 610 31 2.1 0.9 ∘ Example 21 610 35 2.5 615 35 2.6 613 36 2.6 618 36 2.6 0.8 ∘ Example 22 590 28 0.8 593 27 0.8 599 28 0.8 593 28 0.9 0.2 ∘ Comparative Example 23 610 29 1.4 612 30 1.4 614 30 1.5 616 29 1.5 0.2 ∘ Example 24 610 28 1.3 613 29 1.3 615 28 1.4 612 28 1.4 0.0 ∘ Example 25 650 27 1.2 651 26 1.2 650 27 1.2 658 26 1.2 3.0 x Example 26 630 22 0.9 680 21 1.0 687 22 1.0 685 23 0.9 15.0 x Comparative Example 27 630 30 1.3 638 30 1.3 639 31 1.4 640 31 1.3 0.7 ∘ Example 28 630 33 1.4 636 33 1.4 630 33 1.5 638 33 1.5 0.5 ∘ Example 29 630 30 1.3 638 30 1.3 639 31 1.4 640 31 1.3 0.3 ∘ Example 30 630 33 1.4 636 33 1.4 630 33 1.5 638 33 1.5 0.3 ∘ Example 31 630 35 1.8 637 34 1.9 635 35 1.8 633 34 1.9 0.4 ∘ Example 32 600 30 1.8 606 30 1.8 609 30 1.9 600 30 1.8 0.5 ∘ Example 33 600 30 1.8 604 29 1.8 605 31 1.9 601 29 1.9 0.8 ∘ Example 34 820 24 1.6 823 25 1.6 821 25 1.7 825 24 1.7 0.3 ∘ Example 35 820 22 1.6 821 22 1.6 823 23 1.7 830 22 1.7 0.8 ∘ Example 36 695 28 1.8 595 28 1.8 595 28 1.8 595 28 1.8 0.3 ∘ Example 

1. A high-workability steel pipe wherein an r-value in the longitudinal direction is not less than 1.2 over an entire area in the circumferential direction, including a seamed portion.
 2. A method of producing a high-workability steel pipe, said method comprising the step of performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30%, said steel pipe being produced by seam-welding strip steel.
 3. A method of producing a high-workability steel pipe according to claim 2, wherein said method comprises the steps of heating a steel pipe to temperatures of not lower than Ac₁, said steel pipe being produced by seam-welding strip steel, and then immediately or after cooling and reheating said steel pipe, performing diameter-reducing rolling in a temperature range of from 600° C. to Ac₃ with a reduction in diameter of not less than 30%.
 4. A method of producing a high-workability steel pipe according to claim 2 or 3, wherein after the diameter-reducing rolling of said steel pipe, heat treatment of holding the rolled steel pipe in a temperature range of from 600° C. to 900° C. for a time of 1 second or longer is performed during cooling subsequent to the diameter-reducing rolling or by reheating the rolled steel pipe after said cooling. 